How do you convert Weight paint to new mesh layer in blender?
I'm not exactly sure what you are after, but if you want to get a new, separate mesh of the teeth weight painted red, there's a simple way to do it.

In edit mode, you could Circle select (press C, mouse wheel changes brush size) and then "paint" the faces you want to select.

Then Shift-D to duplicate the selection.  Then, P will Separate the selection into a new object.

Hope that helps.